Process of four-colour printing as well as blocks to be used therefor and the printsobtained with said blocks



y 9, 1957 D. .TOLLENAAR 2,798,428

' V PROCESS OF FOUR-COLOUR PRINTING AS WELL AS BLOCKS TO BE USEDTHEREFORE AND THE PRINTS OBTAINED WITH SAID BLOCKS F1166. NOV. 21, 19512 ShetS-Sheet 1 y 9, 1957 D. TOLLENAAR 2,798,428

PROCESS OF FOUR-COLOUR PRINTING AS WELL AS BLOCKS TO BE USED THEREFOREAND THE PRINTS OBTAINED WITH SAID BLOCKS Filed Nov. 21, 1951 2Sheets-Sheet 2 nowmsnucz 1 PROCESS OF FOUR-COLOUR PRINTING AS WELL ASBLOCKS TO BE USED THEREFOR AND THE PRINTS OBTAINED WITH SAID BLOCKSDaniel Tollenaar, Amsterdam,,Netherlands, assignor to StichtingInstituut voor Grafische Techniek T. N. 0., lAnasterdam, Netherlands, acorporation of the Netheran s Application November 21, 1951, Serial No.257,596

Claims priority, application Netherlands November 22, 1950 3 Claims.(Cl. 101-211) As a rule quaternary Levy screens configurations of dotsare used for four-colour printing. According to this process each of thefour colour plates has a screen of identical period. The positions ofthe different screens are taken different to avoid interference. Withthe use of two identical quaternary screens the interference isnegligible small if the angle between the corresponding axes of symmetryof the screens amounts to 30.

During the process of four-colour printing the position of the screensis as a rule such that the screens of the different colour plates forred, blue and black, vary 30, and the screen for yellowis at an angle of15 resp. 75 to the screens for. the blue and the red (sep; aration)colour plates, and at an angle of 45 to the screen for black.

On printing according to this method, a disturbing pattern occurs inorange and in green, as these colours are formed by cooperation of theyellow and the red resp. the yellow and the blue colour plates, therelative screen positions of said colour plates varying only 15. Thisphenomena is often called the 15 pattern. It is visible as small squaresin the oranges and in the greens. When the picture shows much orange theaxis of the screens of black and red may be changed; by this measure thegreat orange areas become free from 15 pattern, but as a rule adisturbing pattern then occurs in other colours of the picture.

The pattern of two systems of parallel lines or of systems of parallelrow of dots, having the equal period d, has a period itself:

2 sin ea in which a is the angle between the corresponding axis of thetwo screens. k becomes d, if a has a value of 60; if on has a value of30, k becomes approximately 1.9 d which means that the interferencepattern in not yet large enough to show up clearly in thecombinationprint of the screens. If a is 15, however, k equals 3.8 d andthe pattern is now clearly visible as a disturbance in thecombination-picture.

An eflfective measure against this 15 pattern is the use of anmezzograph screen in the yellow, in which the screen dots aredistributed at random, so that no interference is possible.

The disadvantage of these screens is that it is exceedingly diflicult toproduce them with the uniformity of density which is required forreproduction. In consequence thereof such prints, as a rule makes acloudy impression; moreover the production of these screens is verycostly.

It has been tried to use 3 quaternary screens, the axes of these screensmutually forming an angle of 30, said 3 screens being combined with onequaternary screen, the axis of which forms an angle of resp. 15, 45 or75 ice with the axes of the first-mentioned screens and by choosing thedot-distance of this latter screen 1.1 times as small as thedot-distances of the other screens. 'By this measure no appreciableimprovement is, however, obtained in comparison to the use of fourscreens with equal dotdistances.

In the German patent specification Number 646,441 it has been proposedto choose for the yellow a screen with double dot-distance and to putthis into the same position as e. g. the screen of the red. This,however, leads to difiiculties when it comes to printing, as, if thismeas:

ure is taken, it is necessary in order to produce an equable colourimpression that the dots of the two screens always have the samerelative location, as shiftings ofonly a few hundredths of a mm. alreadycause a fundamental modi-Q fication in the coincidence of the dots,deviations of colour being the consequence hereof.

The 15 pattern may be avoided by choice of screen positions relativelyvarying 22 30, so that 4 screens may be placed within 90; however, anextremely disturbing configuration of dots occurs if two equal screensare printed at an angle of 45. It is the mean. object of the inventionto avoid the 15 pattern by providing a special combination of screensfor four-colour printings.

It was found, that according to the invention a 15 pattern issubstantially prevented by the use of 3 identical quaternary screens,the axes thereof forming angles of 30 respectively 60 and combining saidscreens with a fourth screen, which has approximately the doubledotdistance, and an axisof which forms an angle .of respectively 15, 45and with the axes of the firstmentioned three screens.

Preferably this deviating screen is used for the yellow as the human eyeis less sensitive to the contrast yellowwhite than to contrasts betweenother colours and white; a yellow print of a certain screen gives theimpression of being much finer if it is compared with a red, 21 blue ora black print of the same screen. Thus, the yellow may be printed in acoarser screen and yet give the same impression as prints made withfiner red, blue or black screens. Only on the use of the fourth screen,the dotdistance thereof being at least 1.5 times as great as thedot-distances of the other three screens, a. considerable improvement inthe pattern is obtained, whereas the pattern becomes almostimperceptible at a dot-distance which is at least twice that of theother three screens.

If however the dot-distance is taken very large, even the yellow givesthe impression of being printed in a coarse screen because each separatedot becomes clearly visible.

Therefore double dot-distance is preferably used for the screen, and adistance more than 2.5 times the dotdistance of the other screens isavoided.

So was obtained as an example a very good result with three quaternaryscreens each having 123 rows of dots per inch, and a fourth quaternaryscreen, having only 63 rows of dots per inch. This fourth screen wasused for yellow, the axis of the screen being horizontal. The axis ofthe red, black and blue screens forming angles with the axis of theyellow screen of respectively 15 45 and 75.

In the annexed drawings is given in Figure l in an enlarged scale apicture, indicating the screens according to this exemple.

In this figure the red screen is indicated with I, the black screen with11, the blue with III and the yellow with IV.

An even better result is obtained if a senary screen is used for theyellow, this screen being placed in a symmetrical position relative tothe screens for red, black and blue, the position of these three screensmutually about twice as large as the period in the other screensAccording to this method the structure of 15 pattern" becomesimperceptibly small.

An example of these screens is given in Figure 2, wherein are indicatedin an enlarged scale three quaternary screens for blue (I), red (II) andblack (III), having 133 rows of dots per inch and a senary Schulzescreen (IV) having 97 rows of dots per inch, and a dot distance of /3the rows distance, the position of t the screens for blue, red, blackand yellow being 15, 75, 45 and.

The invention can be applied for letterpress-printing, for lithoprinting (e. g. oifset plates) as well as for intaglio.

Senary screens are known as such and are occasionally applied'for theprinting of textiles; however, they have not been used in combinationwith three quaternary screens for multicolour printing.

I claim:

1. A process for the production of four-colour prints, usingthreeidentical square screens, the corresponding symmetry axes thereofforming angles respectively of 30 and 60 for printing respectively threecolours and using a fourth hexagonal screen, an axis of which forms anangle of 45 and 75 respectively with an axis of the threefirst-mentioned screens, the dot-distance of said fourth hexagonalscreen being between 1.5 and 2.5 times the dot distances of the otherscreens for printing the fourth colour.

2. Devices such as blocks, offset-plates, intaglio cylinders and platesfor four-colour printing, whereby three of the devices are provided withan identical square screen, the axes thereof forming angles respectivelyof 30 and and a fourth hexagonal screen, an axis of which forms an angleof respectively 15, 45 and with the axes of the other three screens, thedot-distance of said fourth hexagonal screen being between 1.5 and 2.5times the dot distances of the first-mentioned three screens.

3. Four colour prints comprising dots of three colours, said dots beingarranged in square basic nets of a same colour the symmetry of whichmaking mutual angles of 30, and dots of a fourth colour being arrangedin a hexagonal basic net the symmetry of which making angles with saidthree other symmetry axes of 15, 45 and 75 respectively, the dotdistance of the fourth colour being between 1.5 and 2.5 times the dotdistances of the other colours.

References Cited in the file of this patent FOREIGN PATENTS GreatBritain Dec. 12, 1938 Germany June 14, 1937 OTHER REFERENCES

